Abstract
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Objectives 68Ga-labeled radiopharmaceuticals are now used world-wide to evaluate patients with neuroendocrine tumors (NET) and prostate cancer (PC). 68Ga and associated tracers pose unique challenges for scatter correction and can lead to image “wash-out" artifact; in this study we investigated the impact of a modified scatter correction to reduce artifacts in the reconstructed images.
Methods 68Ga imaging presents unique challenges for scatter estimation. Due to the highly-targeted nature of 68Ga-labeled radiopharmaceuticals, even small errors in the scatter estimate can over-power the "true" coincidence counts, pulling regions of the patient background near to or equal to zero. Furthermore, traditional methods of highly down-sampling the emissions image for single-scatter forward-model estimation are insufficient to represent the quickly-changing behavior of the patient activity concentration. Therefore, the modified scatter estimation employs an increased number of down-sampled voxels in the axial direction. Additionally, prompt-gamma correction has been added to the tail fitting step of scatter estimation, accounting for a 1077 keV gamma (that can scatter into the system energy window) and allowing additional fitting flexibility for out-of-field scatter.To analyze the improvement of the proposed scatter estimation, PET datasets from 36 previously acquired clinical research scans [20 68Ga-DOTA TATE and 16 68Ga-RM2 (formerly known as 68Ga-Bombesin or BAY86-7548)] were reconstructed using the novel algorithm and compared to standard reconstruction available on the scanners’ consoles. The data was acquired on three clinical PET systems: GE Discovery PET/CT 610, GE Discovery PET/CT 710, and GE SIGNA PET/MR (GE Healthcare, Waukesha, WI). Each reconstruction was reviewed blindly by a nuclear medicine physician and scored 1-5 (1 poor, 5 excellent quality).
Results Artifacts on 68Ga-DOTA TATE scans were noted in 16 cases for the standard reconstruction and 14 cases for the new scatter correction. The scores for 68Ga-DOTA TATE images were 3.15±1.04 for the standard reconstruction and 3.95±0.89 for the new scatter correction (P <0.0001). Artifacts on 68Ga-RM2 scans were noted in 14 cases for the standard reconstruction and 9 cases for the new scatter correction. The scores for 68Ga-RM2 images were 2.13±1.2 for the standard reconstruction and 4.06±1.06 for the new scatter correction (P <0.0001). In no case for either tracer did an exam’s score decrease with the scatter corrections updates.
Conclusions The new scatter correction algorithm reduces the number and extent of 68Ga-induced artifacts. Visual analysis confirmed that overall image quality is improved as compared to standard scatter estimation.